Delivery delay compensation on synchronised communication devices in a packet switching network

ABSTRACT

The present invention relates to the synchronizing of equipment, and more precisely to the transporting of synchronization signals via a communication network with a view to inter-synchronizing the equipment. The invention concerns a reference station able to deliver packets in a packet switching network to communication devices connected to the network. According to the invention, the reference station comprises means for inserting at least a temporal offset in said packets, wherein said temporal offset describes data delivery duration on a pre-determined path of said network. The invention relates also to a sending communication device and to a receiving communication device.

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/EP2009/054922, filed Apr. 23, 2009, whichwas published in accordance with PCT Article 21(2) on Nov. 5, 2009 inEnglish and which claims the benefit of European patent application No.08300197.4, filed Apr. 30, 2008.

FIELD OF THE INVENTION

The invention relates to the synchronizing of equipment, and moreprecisely to the transport of synchronization signals via acommunication network with a view to inter-synchronizing the equipmentand delivery delay compensation on communication devices connected to apacket switched network.

BACKGROUND OF THE INVENTION

Certain equipment, such as for example video equipment (for examplecameras or video recorders), is able, once synchronized with respect toa reference time base, of providing synchronized data (defining forexample video images). This synchronization is done by transmitting tothe equipment a synchronization signal, which is for example called“Genlock” in the (non-limiting) case of video equipment.

When the transmission of the synchronization signal is done by means ofa dedicated cable, for example of coaxial type, no other signal flows inthis cable. Consequently, the lag in transmitting the synchronizationsignal up to the various items of equipment to be synchronized isconstant and devoid of jitter. On the basis of the synchronizationsignal received, each receiver item of equipment is able to reconstructa timing clock (or reference time base or else reference clock signal)specific to its operation and guaranteeing that each data set (such asan image) that it generates is strictly in phase with each data setgenerated by each other item of equipment which is the subject of thesame synchronization. Thus, two cameras can for example generate videocontents which differ but are strictly in phase and in frequency withrespect to one another. It is recalled that the phase and frequency of aclock constitute what is called its timing.

When the equipment is connected to a communication network introducingtransmission lags and jitters that vary from one item of equipment toanother, as is the case in particular for a so-called packet switchingnetwork, such as a wire-based (Ethernet) or non-wire-based IP network,it is no longer possible to transmit the synchronization signal. Asampled ramp signal which is representative of the synchronizationsignal is then transmitted on the network part.

More precisely, on the send side, information making it possible torecover a reference clock signal and content ticks (for example imageticks) is extracted from the synchronization signal (for exampleGenlock), which is delivered by a master item of equipment. Thereference clock signal supplies first and second counters deliveringfirst and second synchronous ramp signals representative of the numberof ticks of the reference clock signal that have been deliveredrespectively since the last recovered content tick and since a lastreference tick. The value of the first counter is set to zero each timethat a content tick is recovered. A third counter meters the number ofzero settings of the first ramp signal and generates a reference tickeach time that this number is equal to a chosen threshold. The value ofthe second counter (generally termed “PCR” (for “Program ClockReference”)) is set to zero each time that a reference tick isgenerated. The second ramp signal is sampled according to a samplingfrequency (which is generally provided by the network) and the resultingsamples are transmitted via the network to the receiver items ofequipment by means of frames of packet(s).

It will be noted that in the case of video contents, the period of thecontent ticks is for example equal to 40 ms in the case of a 625 linesstandard.

On the receive side, the samples received via the network are used tosynchronize a phase-locked loop (or PLL) and therefore to reconstructthe starting reference clock signal. More precisely, the phase-lockedloop reconstructs the reference clock signal on the basis of the secondsampled ramp signal transmitted, then it reconstructs a second rampsignal identical to, and in phase with, that having been sampled sendside. Processing means are charged with initializing the value of asynchronization counter, synchronous with respect to the secondreconstructed ramp signal, on the basis of the latter and of thereconstructed reference clock signal.

The second ramp signals (PCR), sampled send side at regular intervals(sampling period) arrive at irregular intervals on receive side,predominantly because of the jitter which is introduced by transportingthem (for example over IP). These PCR signals are again taken intoaccount at regular intervals (T_(samp)) on the receive side. Thephase-locked loop (PLL) is charged with filtering the jitter related tothe sampling instant of the PCR counting ramp by the sampling signal ofperiod T_(samp), so that a second ramp signal which evolves strictly ina manner synchronous with the second ramp signal (PCR) generated sendside is retrieved as output from the counter (receive side). Theinaccuracy between the sampling instants on send side and on receiveside is absorbed by the PLL whose bandwidth is appropriate. Thereforethe timing of the reference clock signal reconstructed receive side isidentical to that generated on send side, both in frequency and inphase.

Once the timing has been reconstructed, it is necessary to reconstructthe first ramp signal and synchronize it with respect to the secondreconstructed ramp signal. Accordingly, the same zero-setting period isused as that used on the send side (for example 40 ms). Then, each timethe first ramp signal is set to zero a content tick is generated, andthe initial synchronization signal is reconstructed on the basis of thecontent ticks and the reference clock signals reconstructed.

This synchronization mechanism is disclosed in international applicationPCT FR2007/050918.

By doing this way, equipments driven by such a reference station producesynchronously data streams. But the system described above doesn't copewith the data delivery duration after the stream production which can bea problem. This problem appears for example when a sound and a videosignals are both simultaneously acquired by a microphone and a camerasynchronized as described above. Both, video and audio streams areacquired and delivered synchronously. When such streams are sent to a TVset, if delivery duration of the video stream between the camera and theTV set isn't exactly the same than the delivery duration between themicrophone and the TV set, an audio lip sync effect appears on the TVset caused by the delay between the two streams. The reconstruction ofsynchronization signals on camera and on microphone is not a guaranteethat both streams are perfectly in phase at TV set level.

Another illustration of the problem is encountered when two videostreams delivered by two synchronized cameras are sent to a video mixer.Due to a difference of delivery duration, for example caused by adifference of transport and/or processing duration on two different datapaths, one cannot make a clean transition between two video streams.

One of the goals of the present invention is to remedy the concernsconnected with the delays due to various data delivery duration, whendata streams are produced between various synthesized synchronizationsignals in the prior art, by providing a mechanism that compensate thesedelays.

SUMMARY OF THE INVENTION

The technical problem that present invention intends to solve is totransmit over the network information which can be used to reconstructsynchronization signals in phase on all receive sides.

Thus, the present invention concerns, according to a first aspect, areference station able to deliver packets in a packet switching networkto communication devices connected to the network, said referencestation comprising:

-   -   means for retrieving image ticks from a sync signal;    -   means for initializing an image counter from those image ticks;    -   means for initializing a counter CPT_PCR at each m^(th) reset of        the image counter, the counter CPT_PCR being cadenced by a clock        produced by the image counter;    -   means for sampling the counter CPT_PCR at each T_(ech) period,        T_(ech) being issued by a time base synchronized on all stations        of the network, and    -   means for delivering packets comprising samples PCR_(e) of the        counter CPT_PCR,

To this end, the reference station further comprises means for insertingat least a temporal offset in said packets, wherein said temporal offsetdescribes data delivery duration on a pre-determined path of saidnetwork.

The present invention concerns, according to a second aspect, areceiving communication device connected to a packet switched network,said receiving device being able to receive packets from a referencestation connected to the network, said receiving device being able toreceive data stream from emitting communication devices being connectedto the network, said receiving communication device comprising:

-   -   means for receiving packets comprising samples PCR_(r), said        samples PCR_(r) resulting from a sampling operation realized at        a T_(ech) period, T_(ech) being issued by a time base        synchronized on all stations of the network;    -   means for generating a counter ramp CSR_PCR with a Phase Locked        Loop PLL₁ receiving the samples and delivering a synthesized        clock CLK_out₁ and local samples PCR_loc₁ at each T_(ech)        period;    -   means for initializing an image counter CPT cadenced by the        synthesized clock CLK_out₁ each time the counter ramp CSR_PCR₁        is equal to zero;    -   means for generating image ticks each time the image counter CPT        is equal to zero;    -   means for generating a sync signal from the said image ticks;

To this end, the receiving communication device comprises:

-   -   means for receiving at least a temporal offset value inserted in        said packets;    -   means for receiving data stream from sending communication        device;    -   means for determining from received data stream which temporal        offset it has to consider among received temporal offsets;    -   means for shifting temporally the received data stream with        considered temporal offset.

The present invention concerns, according to a third aspect, a sendingcommunication device connected to a packet switched network, saidsending device being able to receive packets from a reference stationconnected to the network, said sending device being able to send datastream over the network, said sending device comprising:

-   -   means for receiving packets comprising samples PCR_(r), said        samples PCR_(r) resulting from a sampling operation realized at        a T_(ech) period, T_(ech) being issued by a time base        synchronized on all stations of the network;    -   means for generating a counter ramp CSR_PCR with a Phase Locked        Loop PLL₁ receiving the samples and delivering a synthesized        clock CLK_out₁ and local samples PCR_loc₁ at each T_(ech)        period;    -   means for initializing an image counter CPT cadenced by the        synthesized clock CLK_out₁ each time the counter ramp CSR_PCR₁        is equal to zero;    -   means for generating image ticks each time the image counter CPT        is equal to zero;    -   means for generating a sync signal from the said image ticks;

To this end, the sending communication device comprises:

-   -   means for sending data stream over the network synchronously to        sync signal;    -   means for inserting an identification of said sending        communication device in said data stream.

The present invention concerns, according to a fourth aspect, asynchronization transport container for transferring a sample of acounter ramp realized at a frequency 1/T_(ech) between two communicationdevices connected over a packet switched network, T_(ech) being issuedby a time base synchronized on all communication devices.

To this end, the synchronization transport container further comprisesat least a temporal offset which describes data delivery duration on apre-determined path of said network.

Advantageously, the receiving communication device further comprisesmeans for shifting temporally the generated sync signal with consideredtemporal offset.

A first advantage of the invention relates to the ability it gives toinsure a simultaneous phase control of streams sent by multiple sendingcommunication devices to a receiving device similar to those disclosedin international application PCT FR2007/050918. In particular, the PCRramp which is used in current invention is unchanged in regard with theramp signals described in this previous application.

A second advantage of the invention relies on the versatility of theprotocol used to recover the reference signal. It can be implementedwith little adaptation on other types of network infrastructure as itis, and could also be adapted in an industrial automotive system or evenin a WAN environment.

A third advantage of the invention is that it is an easy solution tohandle compensation of flows delay caused by multiple data paths. Due totheir length these delays can not be compensated only by a system ofreducing the network delay and jitter effects belonging to the state inthe art.

A fourth advantage of the invention is that it is a faithful solution.Here, “faithful” highlights the fact that the temporal offset valueswhich are regularly distributed could be modified in time to take intoaccount changes in the production environment (addition or removal ofnew communication devices).

Certain aspects commensurate in scope with the disclosed embodiments areset forth below. It should be understood that these aspects arepresented merely to provide the reader with a brief summary of certainforms the invention might take and that these aspects are not intendedto limit the scope of the invention. Indeed, the invention may encompassa variety of aspects that may not be set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and illustrated by means of thefollowing embodiment and execution examples, in no way imitative, withreference to the appended figures on which:

FIG. 1 represents a high-level view of a video production environmentaccording to the invention;

FIG. 2 shows an example of synchronization transport container structureaccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 describes a video production environment where different blocksmay be listed depending on their role in a flows management:

-   -   Content Delivery Area, 11;    -   Audio Processing Area, 12;    -   Video Processing Area, 13;    -   Content Delivery Area & Monitoring Area, 16;    -   Processed Contents Monitoring Area, 17;    -   Processed Flows Recombination & Storage Area, 18;

All these blocks have network interfaces such as: Video EthernetInserter VIN, 1, 5; Audio Ethernet Inserter AIN, 1, 4; Video EthernetExtractor VEX, 2, 6, 7, 8; Audio Ethernet Extractor, AEX 2, 6, 7, 8.Links between interfaces and network are depicted by arrows in largedotted lines in FIG. 1. The direction of arrow indicates the directionfollowed by the flows between blocks.

Content Delivery Area gathers equipments which deliver contents in thestudio. These equipments belong to various types: camera, microphones,DMOD, VTRs, etc. . . . . They can be located everywhere in a videoproduction studio or even part of equipment used for complementaryfunctions as storage e.g. servers are located in the Content DeliveryArea for the signal feeding part and in the Content Delivery Storage &Monitoring Area and Processed Flows Recombination & Storage Area for thesignal recording part.

Audio Processing Area gathers all the equipments related to audioprocessing, and Video Processing Area gathers all the equipments relatedto video processing.

The block Reference Area, 10 supplies means to synchronize all theequipments belonging to various blocks according the operation modedescribed above.

Based on the destination of the incoming data flows, the differentblocks may run with a different synchronization signal in order tooptimize data delivery delay to their outputs. For an operation point ofview, it is difficult to display simultaneously information coming fromContent Delivery Area and Video Processing Area because an operatorneeds to visualize incoming streams before applying a processing on.

At the level of Processed Flows Recombination & Storage Area, it ismandatory to provide a relevant and efficient solution to manage thedata flow association without issues, for example to avoid audio lipsync effect as explained above.

Interfaces of blocks can be identified by a unique index later called“Genlock Plane”. FIG. 1 shows eight Genlock planes: Genlock Plane 1identifies outputs of Content Delivery Area, Genlock Planes 2 and 4 markthe boundary of the Audio Processing area, Genlock Planes 3 and 5 markthe boundary of the Video processing area; Genlock Plane 6 correspondsto the input of the Content Delivery Storage and Content DeliveryMonitoring Area; Genlock Plane 7 and 8 corresponds to the input of theprocessed Content Monitoring Area, Processed Flows Recombination andstorage area.

The Genlock plane defines a timing of the start of frame of ananalogical Genlock signal which is considered as a reference signal forequipments external to the Network. By construction, all the streamssent by a Genlock Plane are synchronized.

The FIG. 1 reports two different paths of the delivered flows on thevideo production environment. A first flow (audio stream) (representedin plain line) starts from interface 1, crosses blocks 2, 12 and 4 andreaches interface 7 via the network. A second flow (for example a videoHD stream) (represented in dashed line) starts from interface 1 andcrosses blocks 3, 13 and 5 before reaching block 7 still via thenetwork.

Considering that both flows are sent synchronously because they are sentby a same Genlock Plane, they won't exactly arrive at the same date oninterface 7 due to the difference between blocks they cross. Consideringfor example that:

-   -   the network brings a delay of 1 milli-second (ms) to the flows        delivery,    -   the audio interfaces 2 and 4 did not bring any delay to the        flows delivery;    -   the block 12 (internal audio processing) brings a delay of 50 ms        to the flows delivery;    -   the interfaces 3 and 5 (internal video processing) bring a delay        of 50 ms to the flows delivery;    -   the block 13 (video processing) brings a delay of 50 ms to the        flows delivery.

If both flows are sent at date 0, the second flow will arrive atinterface 7 at a date 1+50+50+50+1=152 ms. That means the second flowwill arrive later than the first flow and more precisely with a delay of100 ms. This delay is due to the topology and the configuration of thenetwork: it is perfectly predictable, and then it can be compensated.

A solution to realize easily and faithfully the delivery delaycompensation consists in transmitting regularly values of temporaloffsets (or delay values) brought to data sent by different GenlockPlanes, and in parallel to associate the emission of data with anidentification of the Genlock Plane from which the data is sent.

By receiving these two pieces of information, a receiving communicationdevice can determine which temporal offset it has to consider among thereceived temporal offsets and then shift temporally the received datastream in order to temporally align them.

The feature “easy” of the solution is linked with the chosen way totransmit the temporal offsets: one knows emitting/receiving deviceswhich are able to synchronize themselves by using a 1588 layer andtransmitting samples of a predefined ramp signal in synchronizationmessages. The chosen solution deals with transmitting temporal offsetinside synchronization messages (delivered packet) which contain thesesamples as reminded above.

The feature “faithful” of the solution is linked with its adaptabilityto the “topology” of the network. A modification of the videoenvironment is taken into account by a modification of the temporaloffset values which are sent.

Advantageously, every delivered packet contain temporal offset.

Then, when T_(ech) is equal to one second, a modification can be takeninto account in less than one second.

FIG. 2 shows an implementation of synchronization message delivered overthe network by reference station according to the invention. Thismessage shall be processed by all the communication devices whichtransport or deliver signals coming to or from IP streams.

In the implementation according to the invention, the synchronizationmessage contains a first field corresponding to the value of ramp PCRcounter at the instant T_(ech).

A second field, called “Extension Type”, is also used to indicate whatdata contains the extension and how it is organized. This field existsonly once in the extension. Extension type is a 1 byte field.

Advantageously, the temporal offset is expressed as a number of periodsof a clock common to reference station and to all communication devicesconnected to the network.

A third field, called “Reference Format”, is also used to describe thesignal delivered by the reference station. This field is for exampledivided in 2 bytes: one concerning video reference, another concerningaudio reference. It allows generating at the reception side a referencesignal which has same format than the reference signal received atsending side. For video reference, the following information must bereported: an information on frame rate of the reference signal (50 Hz,60 Hz, 25 Hz, etc. . . . ), an information on format of the referencesignal (for example PAL-BGHI, PAL M, PAL N, NTSC-M, TLS, etc. . . . ),an information on type of signal (SD, HD); and an information ondescription of the signal for example (1920*1080 I, 1920*1080 P, etc. .. . .)

A fourth field, called “Genlock plane #” is also used to indicate anumber of temporal offsets which is enumerated in the currentsynchronization message. “Genlock plane #” is a 1 byte field. 256 is themaximum number of temporal offset (or “Genlock planes”) which can bedescribed in a synchronization message.

At least, if n is a value which is in “Genlock plane #” field, there aren fields called “offset value”. These “offset value” fields indicate thetemporal offset values expressed in a number of reference clock periods(8 bytes field decomposed in two 4 bytes words). These temporal offsetvalues have to be applied to generate the reference signal associated tothe above Genlock plane #.

The invention claimed is:
 1. A reference station configured to deliverpackets in a packet switching network having a non-constant transmissiontime to communication devices connected to the network, said referencestation comprising: means for retrieving image ticks from a sync signalreceived by said reference station; means for initializing an imagecounter from said image ticks; means for initializing a counter at eachm^(th) reset of the image counter, the counter being cadenced by a clockproduced by the image counter, m being a positive, non-zero integer;means for sampling the counter at each T_(ech) period, T_(ech) beingissued by a time base synchronized on all stations of the network, meansfor delivering packets comprising samples of the counter; and, means forinserting at least a temporal offset in said packets, wherein saidtemporal offset describes a data delivery duration on a pre-determinedpath of said network in order to address the non-constant transmissiontime of said packets.
 2. Reference station according to claim 1, whereinevery delivered packet contain temporal offset.
 3. A receivingcommunication device connected over a packet switching network having anon-constant transmission time, said receiving communication devicebeing able to receive packets from a reference station connected to thenetwork, said receiving communication device being able to receive datastream from Content Delivery device being connected to the network, saidreceiving communication device comprising: means for receiving packetscomprising samples, said samples resulting from a sampling operationrealized at a Tech period ° Tech being issued by a time basesynchronized on all stations of the network; means for generating acounter ramp with a Phase Locked Loop receiving the samples anddelivering a synthesized clock and local samples at each Tech period;means for initializing an image counter cadenced by the synthesizedclock each time the counter ramp is equal to zero; means for generatingimage ticks each time the image counter is equal to zero; means forgenerating a sync signal from the said image ticks; means for receivingat least a temporal offset value inserted in said packets in order toaddress the non-constant transmission time of said packets; means forreceiving data stream from sending communication device; means fordetermining from received data stream which temporal offset it has toconsider among received temporal offsets; and means for shiftingtemporally the received data stream with considered temporal offset. 4.Receiving communication device according to claim 3, further comprisingmeans for shifting temporally the generated sync signal with consideredtemporal offset.
 5. A sending communication device connected over apacket switching network having a non-constant transmission time, saidsending communication device being able to receive packets from areference station connected to the network, said sending communicationdevice being able to send data stream over the network, said sendingdevice comprising: means for receiving packets comprising samples, saidsamples resulting from a sampling operation realized at a Tech period,Tech being issued by a time base synchronized on all stations of thenetwork; means for generating a counter ramp with a Phase Locked Loopreceiving the sample and delivering a synthesized clock and localsamples at each Tech period; means for initializing an image countercadenced by the synthesized clock each time the counter ramp is equal tozero; means for generating image ticks each time the image counter isequal to zero; means for generating a sync signal from said image ticks;means for sending data stream over the network synchronously to the syncsignal; and means for inserting an identification of said sendingcommunication device in said data stream in order to address thenon-constant transmission time of said data stream.
 6. The referencestation according to claim 1, wherein the sync signal comprises aGenlock signal.
 7. The reference station according to claim 1, whereinthe non-constant transmission time comprises jitter introduced bytransportation over the network.
 8. The reference station according toclaim 7, wherein the network comprises an IP network.
 9. A method ofdelivering packets in a packet switching network having a non-constanttransmission time to communication devices connected to the network,said reference station comprising: retrieving image ticks from a syncsignal received by said reference station; initializing an image counterfrom said image ticks; initializing a counter at each m^(th) reset ofthe image counter, the counter being cadenced by a clock produced by theimage counter, m being a positive, non-zero integer; sampling thecounter at each T_(ech) period, T_(ech) being issued by a time basesynchronized on all stations of the network, delivering packetscomprising samples of the counter; and inserting at least a temporaloffset in said packets, wherein said temporal offset describes a datadelivery duration on a pre-determined path of said network in order toaddress the non-constant transmission time of said packets.
 10. Themethod according to claim 9, wherein every delivered packet containstemporal offset.
 11. The reference station according to claim 9, whereinthe sync signal comprises a Genlock signal.
 12. The reference stationaccording to claim 9, wherein the non-constant transmission timecomprises jitter introduced by transportation over the network, andwherein the network comprises an IP network.
 13. A method of receivingcommunication over a packet switching network having a non-constanttransmission time, said method comprising: receiving packets comprisingsamples, said samples resulting from a sampling operation realized at aT_(ech) period ·T_(ech) being issued by a time base synchronized on allstations of the network; generating a counter ramp with a Phase LockedLoop receiving the samples and delivering a synthesized clock and localsamples at each T_(ech) period; initializing an image counter cadencedby the synthesized clock each time the counter ramp is equal to zero;generating image ticks each time the image counter is equal to zero;generating a sync signal from the said image ticks; receiving at least atemporal offset value inserted in said packets in order to address thenon-constant transmission time of said packets; receiving data streamfrom sending communication device connected to the network; determiningfrom received data stream which temporal offset it has to consider amongreceived temporal offsets; and shifting temporally the received datastream with considered temporal offset.
 14. The method according toclaim 12, further comprising: shifting temporally the generated syncsignal with considered temporal offset.
 15. The method according toclaim 12, wherein the sync signal comprises a Genlock signal.
 16. Themethod according to claim 12, wherein the non-constant transmission timecomprises jitter introduced by transportation over the network, andwherein the network comprises an IP network.
 17. The method according toclaim 12, further comprising: receiving regularly a plurality oftemporal offset values each being associated with a Genlock plane; andreceiving an identification of the Genlock plane from which the data issent in parallel with the data stream; and determining a temporal offsetto consider among the plurality of temporal offset values received basedon the identification of the Genlock plane.
 18. A method of sendingcommunication over a packet switching network having a non-constanttransmission time, after receiving packets from a reference stationconnected to the network, said method comprising: receiving packetscomprising samples, said samples resulting from a sampling operationrealized at a T_(ech) period, T_(ech) being issued by a time basesynchronized on all stations of the network; generating a counter rampwith a Phase Locked Loop receiving the samples and delivering asynthesized clock and local samples at each T_(ech) period; initializingan image counter cadenced by the synthesized clock each time the counterramp is equal to zero; generating image ticks each time the imagecounter is equal to zero; generating a sync signal from said imageticks; sending data stream over the network synchronously to the syncsignal; and inserting an identification of said sending communicationdevice in said data stream in order to address the non-constanttransmission time of said packets.
 19. The method according to claim 17,wherein the sync signal comprises a Genlock signal.
 20. The methodaccording to claim 17, wherein the non-constant transmission timecomprises jitter introduced by transportation over the network, andwherein the network comprises an IP network.
 21. The method according toclaim 17, wherein the identification of the sending communication deviceenables a receiving device to select a temporal offset among a pluralityof received temporal offset values.